Optical communication provides a number of benefits over other forms of communication. For instance, optical signals using optical fibers provide a number of advantages over electrical signals using conductive wires. A few example advantages include increased bandwidth, reduction in signal degradation over long distances, elimination of electromagnetic cross talk and similar interference, increased data security, ease of installation and avoiding ground loop and similar electrical problems.
Moreover, optical signals can potentially generate lower heat during transmission. Heat can be a major problem with high-speed electrical transmissions on dense integrated circuits (ICs).
While optical communication is currently employed for transmission between devices, the devices typically include ICs that are electrical in nature. For instance, a processor implemented using a silicon-based semiconductor produces electrical signals representing a desired communication. To realize the advantages of optical communications, the electrical signals need to be converted to light and transmitted to the optical fiber or other optical transmission media such as optical waveguide. The same is true for the receipt of optical signals by the processor, in that the optical signals are converted to electrical signals.
Silicon is used in many ICs because it has a number of advantageous properties including cheap cost, existing fabrication techniques and factories, ease of fabrication and the extensive knowledge of its properties in the industry. At common optical communication frequencies (e.g., 1.55 micron) silicon is nearly transparent. Thus, while silicon based structures are capable of routing the light using waveguides, they are not very efficient at producing, modulating and detecting optical signals at wavelengths larger than about 1.1 micron.
A specific type of waveguide is a silicon-on-insulator (SOI) waveguide. Modulators and detectors coupled to waveguides are preferred to surface normal configurations in dense interconnect applications and for use with the low available voltage swings from many silicon circuits. Further, SOI waveguides are prevalent and preferred for their ease of manufacturing, very low intrinsic absorption at near infra-red communication wavelengths, and high mode confinement enabling high density and sharp turns. In addition, a substantial know-how exists regarding the use of SOI waveguides, such as methods for coupling to external fibers and for other passive optical functions. Thus, a monolithic integration of active Si compatible optical modulators, detectors and generators with SOI waveguides is desirable. More specifically, the integration of quantum well modulators and detectors (e.g., bulk and quantum well detectors, primarily PIN, but also MSMs) with SOI waveguides would, depending on the application, directly impact the cost structure, performance, reliability, and/or form factor for communication within an integrated circuit, between chips on a board, between boards over a backplane, within a local area network (LAN) (e.g., Ethernet), and over long-haul distances.
Extensive efforts have been made to integrate active optical devices with Si waveguides using hybrid techniques. Examples of such efforts include attaching and aligning individual pre-fabricated detectors, modulators, and lasers in III-V semiconductors to a silicon waveguide. The need for precise alignment of these active components with the silicon waveguide and the additional bonding step necessary to affix them complicate such efforts.
In addition, there have been efforts to grow germanium directly on silicon to be used as detection elements. These and other processes often require multiple, complicated growth steps, including a long-high temperature anneal phase. The processes can lead to significant SiGe inter-diffusion and large thermal budget and poor throughput in a CMOS fabrication. Moreover, the reported defect density of the so-grown germanium is still large, which is detrimental for its performance.
The above and other difficulties have been challenging to the implementation of optical devices for use in a variety of semiconductor applications.